1. Field of the Invention
This invention is related to infrared detectors, particularly detectors which operate at the temperature of liquid nitrogen or less.
2. Description of the Prior Art
The efficiency of many early infrared detection systems was greatly enhanced by placing the detector or array of detectors in a bath of liquid nitrogen. This technique worked well in laboratory situations, but supplying liquid nitrogen to detectors in field environments has proven to be prohibitive in terms of cost, manpower, transportation and storage. Such detectors have now been largely replaced by mechanical cryogenic coolers using low molecular weight gases such as helium.
The detector dewar presently used employ fragile elements in relationship which made assembly and disassembly a hazardous procedure. The low temperature portion of the cooler is in the shape of a finger having a uniform cross-section and a flat end surface to match the back surface of the detector array: e.g. the flat back surface of a semiconductor chip. The detector is mounted in an evacuated ceramic (e.g. glass) or metal and ceramic dewar to insulate the former from the ambient atmosphere. To accomodate the cold finger a reentrant portion of the dewar wall is shaped to match the cold finger. The reentrant portion is only slightly larger in cross-section than the cold finger so that there will be a minimum of convective gas trapped therein after the cold finger is fully inserted and sealed by means of a rubber gasket near the cross-section where the dewar begins to diverge from the finger. The flat end surface of the finger is presented by a highly conductive and preferably an extremely easily deformable end structure of the finger such as described in U.S. Pat. No. 4,190,106 entitled "Optimized Cooler Dewar", granted Feb. 6, 1980 to Howard L. Dunmire and Stuart B. Horn. The finger's flat end portion itself thus does not overstress the complimentary end portion of the reentrant portion of the dewar. However, as the finger is inserted the rigid portions of the finger can come into contact with the reentrant wall with sufficient pressure to cause cracking and loss of vacuum in the latter. Since the dewars are actually complex structures with sealed wiring and gold plated contacts they are very expensive and the above problems is not a trivial one.